Fatty acids labeled with I.sup.123, i.e. the radioactive isotope of iodine having a mass number of 123, which has a half-life of about 13 hours, in the .omega.-position of the fatty acid molecule, are used as radiopharmaceuticals in diagnostic work with human and animal heart muscles and particularly for the localization of cardiac infarction and ischemia, to determine the deposition, obstruction and like processes involving fatty acids in the heart muscle, and hepatic exchanges as well as the metabolism of myocardial cells.
Reference may be made, in this regard, to the proceedings of the Society of Experimental Biology and Medicine, 148, pp. 215-218 (1975) and to CH. Freundlieb et al, Proceedings, XV, Int. An. Meeting of the Society of Nuclear Medicine, Groningen, September 13-16, 1977, and K Vyska et al, Proceedings, XVI, Int. An. Meeting of the Society of Nuclear Medicine, October 24-27, Madrid, 1978.
The iodine-123, with its half-life of 13 hours, is generally produced in a cyclotron and hence is both relatively expensive and comparatively shortlived so that the production of I.sup.123 labeled compounds must be carried out as rapidly as possible, with maximum yield and with a minimum number of manipulations.
It has already been proposed to react radioactive iodine with .omega.-bromo-fatty acids in a liquid phase, using as a reaction medium, a ketone in which the .omega.-bromo-fatty acid is dissolved. The reaction is halogen replacement or exchange. The yields are less than 60% with a preparation time of 3 hours (see H.-J. Machulla et al, J. Nuclear Med. 19, No. 3, 298-302, 1978.) This relatively low yield and lengthy preparation time have restricted interest in this process.
G. Stocklin, in a review article in Int. J. Appl. Radiat. Isotopes, 28, 1977, p. 131 ff., has indicated that .alpha.-iodo-fatty acids can be made from .omega.-bromo-fatty acids especially effectively by halogen exchange in a melt or fused form. The halogen exchange in the melt is carried out by adding the .alpha.-bromo-fatty acid to the active iodide and forming the resulting mixture. This method has been found to require an excess of the .alpha.-bromo-fatty acid or at least a large quantity thereof to pick up all of the residual iodide resulting from evaporation and bring it into effective contact with the fatty acid.
When one attempts to utilize this technique for the production of .omega.-I.sup.123 fatty acids, correspondingly satisfactory results are not obtained. In fact, the yield of the desired product is materially reduced because a decomposition reaction apparently competes with the exchange reaction which forms the .omega.-I.sup.123 fatty acid and some active iodine is lost by evaporation.